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It takes 120 trillion cells to make a human.
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They are the fundamental units of life,
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making up our brain, muscles, organs - every part of us.
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In the last decade,
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scientists have been able to witness what once seemed impossible -
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the world inside a human cell.
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When I was a student,
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the idea that we could burrow deep inside a living cell was unthinkable.
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Recent advances have made it so scientists can see inside cells
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like never before.
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We can see the parts of single cells and how they work together.
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The more we learn about the universe, the simpler it seems.
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But the cell isn't like that.
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The more we find out, the more complicated things get.
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But these beautiful worlds are also on the front line of the longest war
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in history.
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This is a battle that goes back into the depths of time,
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to a time when the earth was dominated by single cells and viruses.
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Every day, our cells confront these ancient virus enemies,
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tiny, ruthless machines that kill to reproduce.
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There is this whole mechanism inside cells that are taking out viruses
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that previously we just didn't know was there.
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It is a four-billion-year-old struggle that has changed the course
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of our evolution.
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This battle of these viruses against your cells,